The present invention relates to a process for the preparation of olefin polymer foams and, in particular, to an extrusion foaming process for olefin polymers using a stability control additive in conjunction with a blowing agent mixture.
In addition, the present invention also relates to an expandable olefin polymer composition containing the indicated stability control additive and mixed blowing agent system admixed therein and to polyolefin foams prepared therefrom.
It is well known to prepare olefin polymer foams by heat plastifying a normally solid olefin polymer resin, admixing such heat plastified resin with a volatile blowing agent under heat and pressure to form a flowable gel and thereafter extruding the gel into a zone of lower pressure and temperature to expand and cool the gel to form the desired solid olefin foam product.
Until very recently, there had been used only one blowing agent (i.e., 1,2-dichlorotetrafluoroethane) providing sufficient dimensional stability during the curing period.
However, a problem frequently encountered in preparing foams especially of very low density employing 1,2-dichlorotetrafluoroethane as a blowing agent is a limitation on the cross-sectional size of foamed product having smooth skin. This problem occurs when the die opening is increased to provide foam having a larger cross-section because the decrease in hydrostatic pressure in the die causes prefoaming with the result that the extruded foam has a rough skin and/or is physically deformed.
More recently, certain technology has been developed in the area of stability control agents in an attempt to permit the obtention of commercially acceptable dimensional stability with a wider range of volatile halogenated hydrocarbon blowing agents. (See, for example, U.S. Pat No. 3,644,230 and U.S. Pat. No. 4,214,054). Unfortunately, these techniques, while generally providing low density ethylenic polymer foams having improved dimensional stability, are also somewhat limited in terms of the maximum cross-section of the foam articles that can be obtained therewith from a given type of extrusion apparatus. Accordingly, it would be highly desirable to provide an improved extrusion foaming process capable of producing low density olefin polymer foam articles having both relatively large cross-sectional areas (e.g., providing an increase in the maximum achievable foam cross-sectional area which can be obtained with a given extrusion apparatus) and good dimensional stability as well as other desirable foam properties such as closed cells of small cell size and the like.